The polymerization of .alpha.-olefins, especially ethylene, in the gaseous phase or in hydrocarbons as diluents and dispersants, with the aid of solid catalysts that contain elements of IV and/or V sub-group(s) of the Mendeleev Periodic Table, magnesium, halogen, oxygen and, optionally, boron or aluminum, and that have been activated with an alkyl aluminum, is known.
Because the properties of the polyolefins thus prepared are very strongly dependent on the catalyst used, various catalyst systems have already been proposed for the polymerization of the .alpha.-olefins.
In the polymerization of .alpha.-olefins, especially ethylene, the majority of the previously known solid catalysts activated with an alkyl aluminum yield products having a narrow molecular-weight distribution, that is to say, the mean polymerization degree of a polyolefin mixture produced in a polymerization batch lies within a narrow range. This narrow molecular-weight distribution of the polymerizate means that extruded products prepared therefrom have rough surfaces as a result of "melt fractures", so that these polymerizates are unsuitable for many uses, for example, for the manufacture of hollow articles and sheet materials.
The problem was, therefore, to manufacture polyolefins having a broad molecular-weight distribution.
Of the few known catalyst systems that yield products having a broad molecular-weight distribution, the majority cannot be used on an industrial scale because the grain properties of the polymerization product do not satisfy the requirements of the polymerization, working-up or further processing, or because the molecular-weight distribution is not sufficiently broad for many fields of application.
A further group of these catalyst systems has an unsatisfactory catalyst activity so that relatively large quantities have to be employed which pass into the polymerizate, impair its properties, especially its stability, and, therefore, have to be rendered harmless or removed in complicated subsequent processes.
Another group of these known catalyst systems yields products having broadly distributed molecular weights only when the polymerization is carried out in the presence of special activator mixtures, which have to be adhered to accurately, and another group only when the polymerization is carried out in the presence of additional auxiliaries, which subsequently have to be removed from the polymerization medium.
Another group of such known catalyst systems is unsuitable for use on a commercial scale because the physical properties, such as rigidity, impact strength and stress crack resistance, of the shaped articles produced from the polymerization product are inadequate for many purposes.
Some catalysts also discolor the product. These undesirable discolorations are caused chiefly by residual traces of catalysts, which, moreover, in many cases impair the stability of the product to light.
Reference is made to German published application DE-OS 26 35 298, corresponding to U.S. Pat. No. 4,064,334, for an example of a previously known catalyst. This catalyst is produced by grinding a mixture of magnesium halide, an alkoxy aluminum halide, a tetravalent titanium compound and a trivalent titanium compound. However, wall deposits are formed during polymerization, and the polymerizates themselves have a poor powder flow behavior, a low bulk density and a high proportion of fines.